This invention relates to electrostatic spray systems. More specifically, this invention relates to an improvement over the electrostatic coating apparatus disclosed in U.S. Ser. No. 877,445 filed Feb. 13, 1978, in the name of J. Kennon, now U.S. Pat. No. 4,182,490 and my co-pending application U.S. Ser. No. 971,514, filed Dec. 20, 1978, now U.S. Pat. No. 4,273,293, both assigned to the assignee of this invention.
In conventional electrostatic spray coating systems, a fluid coating material such as paint, varnish, lacquer and the like is projected toward an object to be coated in an atomized or particulate form from a dispensing device. The object to be coated is held at electrically ground potential and either just before, at, or just after being dispensed from the gun, the coating material is imparted an electrical charge so that it will be electrostatically attracted toward the object to be coated.
In such systems, it is important that a uniform, smooth, thin coating be deposited on the surface of the object and that a high percentage of the coating material be deposited. The latter criterion is referred to as the transfer efficiency of the system. The transfer efficiency is related to the efficiency of charging the coating material, and workers in the art are moving toward operating at higher charging voltages, e.g., voltages up to 120 kv. However, the use of such high voltages presents certain problems. That is, when spraying many of the coating materials in use today, including powders, a flammable atmosphere results in the area of the coating operation. The high voltage electrostatic charging circuit through the gun causes energy to be capacitively stored in the metallic components of the gun. Thus, if the gun is brought too close to any grounded object, the possibility arises that a spark will jump between the high voltage circuit in the gun and the grounded object igniting the flammable atmosphere in the coating area. The amount of this capacitively stored energy increases as the square of the voltage.
The aforementioned patent, U.S. Pat. No. 4,182,490, shows an electrostatic spray gun having safer operation including a high-valued resistor in the barrel of the gun and a lower-valued resistor in the nozzle of the gun closely adjacent to a material charging electrode projecting from the nozzle effective in damping out the stored energy except for a small amount due to the electrode itself. The material flow and control system shown there, which is much like that shown in my patent, U.S. Pat. No. 3,747,850, has multiple passageways in the barrel of the gun feeding the nozzle out of which the material is eventually ejected as well as a material flow control valve located well inside the barrel of the gun.